The present invention relates to tools and methods used in drilling and completing oil wells. In particular, the invention relates to a system that can be used to reliably release a drill string coupled to a liner after running the liner into a well, especially an extended reach well.
An extended reach well can be defined as a borehole having a major portion at a high angle to the vertical, typically having a short vertical portion followed by a long, nearly horizontal portion. The result is that the bottom of the well may be relatively shallow (e.g., less than 3,000 feet) but horizontally displaced a great distance (e.g., greater than 5,000 feet) from the top of the well.
When drilling a typical oil well, a rotating drill string and attached drill bit are used in conjunction with a flow of drilling mud to drill a borehole. After the borehole is drilled, a casing may be run in the borehole across unproductive intervals, and/or a liner may be run in across a productive interval. The liner or casing (or other tubulars) may be removably attached to a drill or work string during the run in operation.
Running the liner through an extended reach well can be difficult because of friction between the liner and the borehole wall. A drag force is caused by the weight of the liner and drill pipe bearing against a near horizontal wall of the borehole. The drag force can cause serious problems.
Several methods are known to cope with this drag problem. Increasing the lubricity of the drilling mud has been used to reduce the friction-caused drag during the running of the liner or casing. Running tubulars may also require a flotation cavity as described U.S. Pat. No. 4,986,361 (and incorporated herein by reference) to reduce the effective weight and thereby decrease drag forces. However, the flotation cavity can prevent the flow of drilling muds. Even with a decreased effective weight, running a floated liner dry (without a circulation of drilling mud) can offset any decrease in drag resulting from the decreased effective weight.
Still further, conventional tools to run tubulars into extended reach well holes have not been able to rotate the tubulars while they were being run in. One direction of rotation is conventionally used to release the run in tool from the tubular once it is in place. Thus, oscillations in rotating torques and string weight may cause the conventional connections to release prematurely. Other conventional tools permit rotation after run in, but these tools may require operational interruption and may also not allow fluid (e.g., drilling muds or cement slurry) flow past the tool.
Once the casing or liner is in place, tools which do not allow fluid flow are typically replaced so that a cement slurry slug can be injected to cement the tubular to the wall of the borehole. The cement slurry slug is pumped down through the drill pipe, tools, tubular, and into a borehole annulus between the liner or casing and the borehole wall. The cement slurry slug is typically followed downhole by drilling mud or other pump down fluid. The cement slurry slug is conventionally separated from the pump down fluid by a wiper or drill pipe dart which passes through the drill string, releasing tool and most of the casing or liner. The dart conventionally stops or lands near the bottom of the casing or liner at a landing collar. The collar and landed dart prevent any further displacement of the cement slurry until the cement sets. Once the liner or casing is cemented in place, the drill pipe is uncoupled from the tubular, and the drill pipe is removed from the borehole. Residual cement, float shoe, dart, and float collar can then be removed in a drill out operation prior to production from the well.
Because of these various tool limitations, it has not been generally possible to float, rotate, and flow lubricating fluid during run in of tubulars, and then flow cement slurry, position the slurry, anchor, and cement the tubular without at least interrupting operations and changing tools. In addition, the reliability of the dart passing through near horizontal tools and tubulars to the landing collar may not be as high as desired. Tools, for example, may constrict and trap the dart before passage through the tubular, preventing acceptable slurry fill of the borehole annulus and cementing. This results in inadequate sealing of the borehole annulus as well as a failure to uncouple the tubular and remove the work string.